msm: mdss: reduce vsync waiting time only when it is enabled

Reduce vsync waiting time only when vsync is still enabled.
Otherwise, it will trigger waiting for next vsync mistakenly
and timeout eventually.

CRs-Fixed: 762791
Change-Id: Ic3df12a4b449fa6d6cbbd1169e890b0cf3f67db1
Signed-off-by: Kuogee Hsieh <khsieh@codeaurora.org>